A process for treating water to inhibit scale formation

Abstract: Described is a process for treating water to inhibit scale formation, comprising adding to the water an effective amount of a maleate copolymer having a weight average molecular weight of 500 to 7,000, a residual total monomer content of less than 1%, on a maleate copolymer dry weight basis, and comprising at least 50 mole percent of maleate monomer, whereby at least 70 mole percent of the carboxylic acid charge on the maleate monomer is neutralized prior to polymerization, and 10 to 50 mole percent of acrylate monomer.

Description: This invention relates to a method for treating water to inhibit scale formation, comprising adding to the water an effective amount of maleate copolymer having a weight average molecular weight of 500 to 7,000 and a residual total monomer content of less than 1% based on a maleate copolymer dry weight basis, wherein said maleate copolymer comprises at least 50 mole percent of maleate monomer, 10 to 50 mole percent of acrylic or methacrylic acid, and 1 to 10 mole percent of at least one nonionic monomer selected from the group consisting of acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dipropylacrylamide, and vinyl acetate, whereby the proportions of constituents add up to 100 mole % wherein at least 70 mole percent of the carboxylic acid charge on the maleate monomer is neutralized prior to aqueous solution polymerization, and the maleate copolymer is prepared with a redox catalyst comprising at least one initiator and at least one metallic ion. Copolymers of maleic acid have unique functional properties that are prepared by an aqueous solution process to yields a copolymer comprising at least 50 mole percent maleate and less than 1%, on a copolymer weight basis, residual free monomer. The process used to prepare this copolymer is carried out in the presence of a redox catalyst system, comprising at least one initiator and appropriate metallic ions, with slow feed addition of the comonomer(s), together with the post-addition of initiator. The maleate copolymers are useful in a variety of applications requiring modification of salt crystal formation in aqueous solution. It is known that maleic acid and maleic anhydride monomers do not readily form homopolymers, but will quite rapidly copolymerize with various other ethylenically unsaturated monomers, typically to form 1:1 molar ratio maleate:comonomer copolymers. In most cases, maleate copolymers are prepared by a solution polymerization process employing organic solvents. For example, U.S. Patent No. 4,710,537, issued December 1, 1987 to Walinsky, teaches the preparation of a substantially homogeneous copolymer, consisting essentially of 35 to 65 mole percent acrylate and 65 to 35 mole percent maleate units and having a number average molecular weight of about 500 to 5,000. The copolymer is prepared in a chain-transfer solvent, preferably a ketone, ester, or ether having from 4 to 10 carbon atoms. Following polymerization, an aqueous solution of the copolymer may be prepared by adding water to the polymerization flask and distilling off the organic solvent. For reasons of safety, efficiency in manufacturing and waste handling, it is desirable to polymerize the maleate copolymers in aqueous solution, rather than in organic solvent. An aqueous solution preparation of maleate copolymers is disclosed in JP -A-60-212,410, published October 24, 1985 by Fukumoto, et al. The polymerization method provides maleate copolymers with mean molecular weights of 300 to 5,000 and has a reported "polymerization rate" of, at most, 96.2%. The "polymerization rate" is the difference between the weight of monomer initially charged and the weight of copolymer recovered following polymerization (i.e., a measure of the unreacted residual monomer). Another method for preparation of maleate copolymers in aqueous solution is disclosed in U.S. -A-4,519,920, issued May 28, 1985, to Fukumoto, et al. This process provides an initial, partial neutralization of the maleate monomer with alkali. A similar aqueous solution process for producing a polymaleate, with an option to copolymerize up to 20 mole percent of an unsaturated carboxylic acid comonomer, is disclosed in U.S. -A-4,668,735, issued May 26, 1987, to Fukumoto, et al. U.S. -A-3,635,915, issued January 18, 1972 to Gale, discloses a process for preparing maleate/acrylate copolymers in aqueous solution using an inorganic peracid salt as a catalyst. The copolymers are prepared from 50 to 95 parts of acrylic acid and 5 to 50 parts of maleic acid, on a monomer weight basis (i.e., 57.5 to 96.5-mole percent acrylate and 3.5 to 42.5 mole percent maleate). U.S. -A-4,659,793, issued April 21, 1987, to Yang claims an aqueous polymerization process for manufacturing copolymer having a low residual free monomer content from carboxylic acid monomers and various comonomers. In the claimed process a metallic ion redox catalyst is used to prepare a copolymer comprising 25 to 55 weight percent (i.e., 21 to 47 mole percent) dicarboxylic acid monomer. Further, there is disclosed a maleate copolymer having a weight average molecular weight of 2,000 to 1,000,000, a residual total monomer content of less than 0.5% on a maleate copolymer dry weight basis, and comprising of 10 to 70% of maleate, 10 to 50% of acrylate monomer and not more than 30% vinyl monomers selected from acrylamide, N,N'-dimethylacrylamide, hydroxy ethyl acrylate, hydroxypropylacrylate, vinyl-pyrrolidone and vinyl acetate. Such copolymers are used as antiscalants, dispersants and detergent additives. U.S. -A-4,314,044, issued February 2, 1982 to Hughes, et al., claims an aqueous polymerization process for manufacturing copolymers of carboxylic acid monomers using a metallic ion redox catalyst system. As exemplified in the patent, the process only achieves about 95% monomer conversion. About 5 to 50 weight percent dicarboxylic acid is polymerized in the claimed process. The processes for preparing maleate polymers and copolymers known in the art disadvantageously result in unacceptable levels of free, residual monomer, in particular, free, residual maleate monomer in the product of the polymerization. Such free, residual monomer is undesirable in many end use applications, particularly applications relating to personal care products, detergents and cleaners as well as industrial end uses involving further chemical reactions in which the residual monomer may participate as an undesirable reactant. We have now discovered that an aqueous solution polymerization, employing a combination of at least 70 mole percent charge preneutralization of the maleate monomer, a redox initiator catalyst using metallic ion, a slow feed addition of at least one comonomer, preferably over a period of at least three hours, to the neutralized maleate monomer and a post-feed of a polymerization initiator provides a copolymer having a weight average molecular weight of about 500 to 7,000, comprising at least 50 mole percent maleate, and less than 1%, on a copolymer weight basis, of residual, free monomer. This process may be carried out under ambient atmospheric conditions. An inert atmosphere, such as nitrogen, is not required for the polymerization. The resulting copolymer is useful as an antiscalent, an anti-incrustation agent, a dispersant, a detergent adjunct, a sequestrant and a water hardness salt (e.g., calcium carbonate, magnesium carbonate, magnesium sulfate, magnesium hydroxide, calcium sulfate and barium sulfate) crystal modifier. The processes claimed herein include processes for sequestering water hardness ions, modifying crystal formation in water and inhibiting scale formation (e.g., in circulating water systems and oil field production systems); each utilizing the maleate copolymer of this invention. The maleate copolymer comprises at least 50 mole percent maleate, 10 to 50 mole percent acrylic or methacrylic acid and 1 to 10 mole percent of at least one nonionic monomer selected from the group constisting of acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dipropylacrylamide and vinylacetate, and less than 1%, preferably less than 0.25%, by weight, residual total monomer content whereby the proportions of constituents add up to 100 mole%. The maleate copolymer has a weight average molecular weight of about 500 to 7,000, preferably 2, 000 to 4, 000. The maleate copolymers having unique functional properties are prepared by an aqueous solution polymerization process. At least 70%, on a mole charge basis, of the charged groups of the maleate monomer are preneutralized with alkali. An effective amount (e.g., 1-4 ppm), on a total batch weight basis, metallic ions suitable for use as a reductant in a redox catalyst is added to the preneutralized maleate monomer. A monocarboxylic acid monomer, and at least one initiator, and a third nonionic comonomer, and, optionally, additional alkali, are slowly added to the neutralized maleate over at least a three hour period at 80-180°C. Following a post-addition of more initiator, preferably to provide a total of at least 4% initiator, on a total monomer mole basis, the aqueous solution of the maleate copolymer comprises less than 1%, by weight, free, residual monomer and the copolymer has a weight average molecular weight of about 500 to 7,000. The capacity of the maleate copolymers of this invention to modify water hardness salt crystal formation is illustrated in Figures 1 - 4 herein. Figures 1 - 4 are photomicrographs of calcium carbonate crystals formed after treatment with the maleate copolymers and various controls. Figure 4 illustrates the crystal modification properties of the maleate copolymers of this invention. Figures 1 - 3 illustrate the properties of three control polymers. The samples and testing methods are set forth in Example 2, below. In the process for preparing the copolymers of this invention an aqueous solution, comprising 20 to 35%, preferably comprising 27 to 30%, by weight, of maleate monomer is prepared. Suitable maleate monomers include maleic acid, maleic anhydride, alkali metal maleate salts, ammonium maleate salts, and mixtures thereof. In the initial step of the process, the aqueous maleate solution is neutralized by the addition of alkali. Each. mole of maleate comprises two moles of carboxylic acid charge. Thus, two moles of alkali are required for each mole of maleate monomer to provide 100% charge neutralization of the monomer. For the purposes of this invention, at least 70% of the carboxylic acid charge, preferably at least 80%, must be neutralized prior to polymerization. Any base may be used for this step. In a preferred embodiment, an aqueous solution of an alkali metal hydroxide, such as sodium or potassium hydroxide or an aqueous ammonium solution is added to the maleate monomer solution. The polymerization is carried out at 80 to 180°C, preferably 90 to 100°C, and most preferably, 95 to 100°C, under reflux. If the temperature is permitted to remain below 90°C, the neutralized maleate salt is likely to precipitate out of the reaction solution. If a relatively labile comonomer, such as acrylamide, is used, then monomer will be lost at temperatures exceeding 100°C and it is preferred to maintain the temperature below 100°C. The reflux may be carried out under ambient atmosphere or under inert atmosphere, such as nitrogen. Likewise, the reaction may be carried out under greater than atmospheric pressure (e.g., ½ to 1½ atmospheres), although ordinary atmospheric conditions are suitable and are preferred. The process employs as a polymerization initiator at least one redox catalyst, comprising at least one initiator and at least one metallic ion suitable for use as a reductant in a redox catalyst system. Such reducible cations include the metallic ions obtained from iron, zinc, cobalt, molybdenum, chronium, nickel, vanadium and cerium, and combinations thereof. Preferred metallic ions are obtained from ferrous ammonium sulfate, ferrous sulfate, ferrous chloride, cobalt salts (e.g., cobalt sulfate hexahydrate), vanadium salts, and combinations thereof, and other water soluble salts producing metallic ions that are susceptible to oxidation by oxidants typically used in redox catalysts as polymerization initiators. Such initiators include hydrogen peroxide, benzoyl peroxide, sodium persulfate and sodium persulfate bisulfite, t-butyl hydroperoxide, cumene hydroperoxide, dialkyl peroxide, ammonium persulfate and ammonium persulfate/bisulfite, and combinations thereof. A mixture of initiators is preferred. The initiator is preferably utilized at about 4 to 15%, on a total monomer mole basis, together with an effective amount of metallic ion, preferably at least 1 ppm, most preferably at least 4 ppm metallic ion, on a total batch weight basis. The initiator and metallic ion may be added to the initial maleate charge, or added with the feed of the comonomer(s), or both. A reductant, such as erythorbic acid or ascorbic acid is preferably added after polymerization. A post-polymerization addition of initiator is an essential aspect of this invention. In a preferred embodiment an initiator comprising, on a total monomer weight basis, 10 to 15% hydrogen peroxide and 4 to 6% sodium persulfate is added along with the comonomer(s) feed. A total of 7 to 40 ppm ferrous ammonium sulfate hexahydrate is added to the initial maleate charge. A total of 1 to 2% hydrogen peroxide and 0.25 to 0.35% sodium persulfate is added during the post-polymerization period. The comonomer(s), either in aqueous solution or neat, are added slowly during polymerization over a period from about 3 to 10 hours. In a preferred embodiment, the comonomer(s) are partially neutralized by adding the comonomer(s) to an aqueous solution of alkali prior to addition to the polymerization flask. In the alternative, alkali may be added simultaneously with the comonomer(s) to regulate reaction pH. Suitable comonomers include ethylenically unsaturated monocarboxylic acids, preferably acrylic acid, and, optionally, other copolymerizable comonomers. Preferred monocarboxylic acid monomers include acrylic acid, methacrylic acid and ethacrylic acid, and combinations thereof (the "acrylate" monomer). The acrylate monomers useful in the production of the polymers of this invention are the olefinically unsaturated carboxylic acids containing at least one activated carbon-to-carbon olefinic double bond, and at least one carboxyl group, that is, an acid containing an olefinic double bond which readily functions in polymerization because of its presence in the monomer molecule either in the alpha-beta position with respect to a carboxyl group, or as a part of a terminal methylene grouping. In a preferred copolymer of this invention, the copolymer comprises at least one comonomer in addition to the maleate and acrylate monomers. Useful olefinically-unsaturated acids of this class include such widely divergent materials as the acrylic acid comonomers typified by acrylic acid itself, methacrylic acid, ethacrylic acid, alpha-chloro-acrylic acid, alpha-cyano acrylic acid, beta methyl-acrylic acid (crotonic acid), alpha-phenyl acrylic acid, beta-acryloxy propionic acid, sorbic acid, alpha-chloro sorbic acid, angelic acid, cinnamic acid, p-chloro cinnamic acid, beta-styryl acrylic acid (1-carboxy-4-phenyl butadiene-1,3), itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, fumaric acid, and tricarboxy ethylene. For the polycarboxylic acid monomers, an anhydride group is formed by the elimination of one molecule of water from two carboxyl groups located on the same polycarboxylic acid molecule. The preferred carboxylic monomers for use in this invention are the monoolefinic acrylic acids having a substituent selected from the class consisting of hydrogen, halogen and hydroxyl groups, monovalent alkyl radicals, monovalent aryl radicals, monovalent aralkyl radicals, monovalent alkaryl radicals and monovalent cycloaliphatic radicals. Other suitable optional comonomers for preparing the copolymers herein include any water- soluble, unsaturated compounds that are capable of free radical polymerization utilizing a redox catalyst. Water soluble is defined herein to mean comonomers which have a minimum solubility of 5% by weight in water at 25°C. Such comonomers include acrylic and methacrylic acid (the "acrylate" monomer group); acrylamide, methacrylamide, acrylonitrile; N,N-dialkylaminoalkyl acrylates and methacrylates (wherein the alkyl group contains 1 to 4 carbon atoms); ethylenically unsaturated quaternary ammonium salts such as N,N,N-trimethylaminoethyl methacrylate methylsulfate or halide, 2-hydroxy-3-methacryloxypropyltrimethyl-ammonium methyl sulfate or halide, vinylbenzyltrialkylammonium methylsulfate or halide; sodium or ammonium styrenesulfonate; vinyl pyrrolidinone; hydroxyl alkyl acrylates and methacrylates; sodium 2-acrylamide-2-methylpropane sulfonate, etc. Various other water-soluble comonomers suitable for polymerization with maleate and acrylate monomers are known to those skilled in the art. The maleate monomer is present in the copolymers of this invention at a mole ratio of least 50%. The second carboxylic acid monomer, preferably acrylic acid monomer, is present in the copolymer of this invention at a mole ratio of 10 to 50%. The copolymer may further comprise at least one additional non-ionic comonomer, exemplified by acrylamide, N-vinyl-2-pyrrolidinone, diallylamine, hydroxypropylacrylate, hydroxyethylacrylate and vinyl acetate. The copolymer comprises at least 50 mole percent maleate, 10 to 50 mole percent acrylate and 1 to 10 mole percent non-ionic comonomer. The copolymers prepared by the process of this invention preferably have a weight average molecular weight of 500 to 7000. In a preferred embodiment, the copolymers have a molecular weight of 1000 to 4000, most preferably 2000 to 4000. At these molecular weights, the copolymers of this invention exhibit improved performance in controlling water hardness, compared to higher molecular weight (e.g., 15,000-30,000) copolymers comprising maleate/acrylate monomers. The copolymers of this invention are characterized by an extremely low residual monomer content of less than about 1%, on a dry copolymer weight basis. In a preferred embodiment, the copolymers of this invention comprise no more than 0.25% residual monomer, on a copolymer dry weight basis. The percent residual monomer calculations are based on maleate monomer. The copolymers of maleate/acrylate and of maleate/acrylate/comonomer comprising at least 50 mole percent maleate and having a weight average molecular weight of 500 to 7,000 and less than 1%, on a copolymer weight basis, residual monomer, exhibit unique functional characteristics in various end use applications. The copolymers of this invention are useful in quantities greater than 0.5 ppm, preferably at 1 to 100 ppm, most preferably 5 to 30 ppm, on a water weight basis, to control scale formation and sequester water hardness ions in water treatment applications. When the maleate copolymer were evaluated for modification of water hardness salts, copolymers comprising a third comonomer provided superior modification of salt crystal formation and much greater efficacy in controlling water hardness relative to commercially used polymers. In addition, maleate copolymers comprising only maleate and acrylate prepared by the process of this invention showed improved performance in treatment of water hardness, compared to copolymers having a similar monomer molar ratio and molecular weight, but which had been prepared by other polymerization processes known in the art. The maleate copolymer is useful as a water treatment agent in boiler feed water, cooling water, air conditioning water, circulating water (for heating, cooling or maintaining temperature or for insulation), commercial water systems containing water hardness ions, manufacturing operations, and waste water treatment operations. Effective usage levels will depend upon the particular water system to be treated, salt concentration and other variables. The maleate copolymers may be used to treat water used in paper manufacturing, sugar manufacturing, oil field production (e.g., at 100 ppm to 0.5% of the water system) and other industrial applications requiring salt crystal modifications in aqueous solutions, slurries and dispersions. EXAMPLE 1 This example illustrates the process for preparing the copolymers of this invention. Synthesis of the copolymers is described in Parts A - E, below. Copolymer characteristics are described in Table I, below. Copolymer Synthesis Copalymer Sample Monomer Molar Ratio Degree of Pre-Neutralization of Maleate (mole %) Final pH Mw Mn Percent Residual Monomer Part A 1 MA/AA/AM 50/45/5 70 5.2 2522 1805 <0.01 2 MA/AA 60/40 80 5.7 2297 1555 0.1 3 MA/AA 50/50 50 6.3 2672 1254 <0.01 Part B 4 MA/AA/AM 50/40/10 75 5.2 2367 1755 0.03 5 MA/AA/AM 50/42/8 85 5.7 2203 1653 <0.1 6 MA/AA/AM 60/35/15 85 5.8 1786 1113 0.18 7 MA/AA/AM 50/49/1 80 4.9 2700 1906 <0.01 8 MA/AA/AM 50/35/15 80 5.6 2830 1909 0.07 Part C 9 MA/AA/VP 50/45/5 80 5.0 2639 1873 <0.1 10 MA/AA/DA 50/45/5 80 5.2 2990 1876 <0.1 11 MA/AA/HPA 50/45/5 80 5.1 2698 1727 <0.1 12 MA/AA/HEA 50/45/5 80 5.1 2652 1851 <0.1 13 MA/AA/AM 50/45/5 80 5.4 3063 2028 <0.1 Part D 14 MA/AA/VA 80 4.8 3246 2092 <0.01 50/45/5 15 MA/AM 50 5.8 2698 1643 <0.01 70/30 Part E 16 MA/AA 67/33 52.5 - 3600 ― 2.1 (precipitated) Part F 17 MA/AA 67/33 52.5 5.2 3094 1938 0.40 Part A Copolymer 1 (50/45/5 maleic/acrylic/acrylamide): A charge of 2.47 moles (242.0 grams) maleic anhydride in 350 grams of deionized water was placed into a flask equipped with two dropping funnels, a reflux condenser, a nitrogen inlet and a stirrer. The maleic anhydride was 70% neutralized by the addition of 276.6 grams of 50% sodium hydroxide solution (0.7 mole of base/mole of carboxylic acid charge of the maleate monomer, or 1.4 moles base/mole of maleate monomer). Based on monomer weight, 10.6 ppm ferrous ions (0.03 grams of ferrous ammonium sulfate hexahydrate), and 1.0 g ascorbic acid were added to the flask. At a reflux temperature of 98-100°C, a feed containing 2.23 moles (160.6 grams) of acrylic acid and 0.25 moles acrylamide (34.0 g of a 52% aqueous solution) and 30.0 g of a 50% solution of sodium hydroxide in 130.0 grams of deionized water, and a second feed containing 11.8% (142 g) of hydrogen peroxide and 4.3% sodium persulfate (18 g in 104.5 water), on a total monomer weight basis, were added drop wise, with stirring, to the flask over a 5 hour, 15 minute period followed by a two hour cook. Total solids, pH and intrinsic viscosity in 1N NaCl were recorded. The molecular weight was measured by gel permeation chromatography using polyacrylate standards. The residual monomer (a total of maleic acid, acrylic acid and fumaric acid) was measured by liquid chromatography using a Waters Liquid Chromatograph apparatus. Copolymer 2 (67/33 maleic/acrylic): A copolymer was synthesized as described above except: (1) 1.32 moles of maleic anhydride (129.4 grams) and 0.66 moles of acrylic acid (47.5 grams) were used, (2) the initiator charge comprised 10.8% hydrogen peroxide and 4.5% sodium persulfate, on a total monomer mole basis, (3) 5 ppm (on a total batch weight basis) ferrous ammonium sulfate hexahydrate, was added to the maleate monomer, (4) the feed time was 5 hours followed by a 20 minute initiator post feed and (5) a post treatment of 0.17%, on a monomer weight basis, of erythorbic acid was added to the aqueous copolymer. Copolymer 3 (50/50 maleic/acrylic): A copolymer was synthesized as described above, except: (1) a charge of 2.57 moles of maleic anhydride (251.5 g) in 400 g of deionized water 0.118 g ferrous ammonium sulfate hexahydrate, 150 g isopropanol, and 209.5 g of a 50% solution of sodium hydroxide, and a feed of 2.57 moles acrylic acid (184.8 g) and 104.4 g of a 50% solution of sodium hydroxide in 140.1 g of deionized water were used, (2), the maleate was 50% neutralized with base prior to polymerization, (3) comonomer feed was added over a 5 hour period, and an additional initiator (168.5 g of a 35% solution of hydrogen peroxide) feed was added simultaneously over a 6 hour period. Following the synthesis, 85 g of a 50% sodium hydroxide solution, was added to the flask and the copolymer was recovered in aqueous solution following distillation to remove isopropanol. Part B Copolymers 4-8 (maleic/acrylic/acryl-amide): Copolymers were synthesized as described above except that the molar ratio of the monomers was varied. Copolymer 4 was prepared from a molar ratio of 50/40/10 maleic/acrylic/acrylamide, using 2.57 moles maleic anhydride (251.9 g), 2.06 moles acrylic acid (148.3 g) and 0.51 moles acrylamide (69.6 g of a 52% solution). The initial charge included the maleate, 400.0 g of deionized water, 308.4 g of a 50% sodium hydroxide solution, 150.0 g isopropanol, 0.118 g of ferrous ammonium sulfate hexahydrate and 2.5 g of a 35% solution of hydrogen peroxide. The monomer feed contained 150.0 g of deionized water in addition to the acrylic acid and acrylamide. The initiator feed contained 183.0 g of hydrogen peroxide in 100.0 g of deionized water. Following synthesis, 250.0 g of 50% sodium hydroxide was added. Copolymer 5 was prepared from a molar ratio of 50/42/8 maleic/acrylic/acrylamide, using 2.57 moles maleic anhydride (251.9 g), 2.14 moles acrylic acid (154.0 g, glacial) and 0.43 moles acrylamide (58.7 g, 52% solution). The initial charge contained the maleic anhydride, 362 g deionized water 349.5 g of 50% sodium hydroxide, and an initiator mixture of 0.03 g ferrous ammonium sulfate hexahydrate and 3.0 g of ethythorbic acid. The monomer feed contained the acrylic acid, acrylamide, 20.0 g of 50% sodium hydroxide and 175.0 g of deionized water. The initiator feed contained 1.42 moles of hydrogen peroxide (138.0 g of a 35% solution) and 0.122 moles of sodium persulfate (29 g in 116.5 g of deionized water). The monomer was added over 5 hours, the initiator over 5 hours and 15 minutes and synthesis was followed by a 2 hour cook. Copolymer 6 was prepared from a molar ratio of 60/35/5 maleic/acrylic/aerylamide, using 3.08 moles maleic anhydride (301.8 g), 1.80 moles acrylic acid (129.6 g, glacial) and 0.25 moles acrylamide (34.2 g, 52% solution). The initial charge contained the same components as in copolymer 5 except sodium hydroxide was increased to 418.8 g and the water to 385.0 g. The monomer feed contained the monomers in 15.0 g of 50% sodium hydroxide and 120.0 g of water. The initiator feed contained 150.0 g of 35% hydrogen peroxide, and 35.0 g of sodium persulfate in 120.0 g of water. Copolymer 7 was prepared from a molar ratio of 50/49/1 maleic/acrylic/acrylamide, using 1.23 moles maleic anhydride (121 g), 1.21 moles acrylic acid (87.1 g, glacial) and 0.025 moles acrylamide (3.5 g, 50% solution). The initial charge contained maleic anhydride, 175 g deionized water 158 g 50% sodium hydroxide, and 0.02 g of ferrous ammonium sulfate hexahydrate. The monomer feed contained 64.5 g of 35% hydrogen peroxide, and 9.5 g of sodium persulfate in 10 g deionized water. Copolymer 8 was prepared from a molar ratio of 50/35/15 maleic/acrylic/acrylamide, using 1.23 moles maleic anhydride, (121 g), 0.863 moles acrylic acid (62.1 g, glacial) and 0.37 moles acrylamide (52.5 g of 50% solution) . The initial charge contained the same components as Copolymer 7. The monomer feed contained the monomers in 45.0 g of deionized water. The initiator feed was the same as Copolymer 7. Part C Copolymers 9-14 (maleic/acrylic 50/45 and 5% third monomer): Copolymers 9-14 were synthesized as described above for copolymer 3, except that various non-ionic monomers were substituted for acrylamide at 5 mole percent. The third monomers tested included, N-vinyl-pyrrolidone, diallylamine, hydroxypropyl acrylate, hydroxyethyl acrylate, vinyl acetate and acrylamide (control). Part D For comparative purposes, copolymer 15, a copolymer of maleic acid and acrylamide in a molar ratio of 70/30 was synthesized as described for copolymer 3, above, except no acrylic acid was used. Part E For comparative purposes, copolymer 16, a copolymer of maleic and acrylic acids in 67/33 molar ratio, was synthesized as described in Example 1 of U.S. Pat. No. 4,519,920 (the "Kao '920 patent"). In the Kao '920 synthesis, the initial maleate charge was only 52.5% neutralized with sodium hydroxide (a molar ratio of maleic anhydride/sodium hydroxide of 1.00/1.05). The Kao '920 synthesis also differed from the synthesis of copolymer 2 of this invention in that an additional 5.0% sodium hydroxide was added with the acrylic acid, only 9.55%, on a total monomer molar basis, of a single initiator (hydrogen peroxide) was used, and there was no initiator post-feed. Part F For comparative purposes, copolymer 17, a copolymer of maleic and acrylic acids in 67/33 molar ratio, was synthesized as described for copolymer 2, above, except the maleic anhydride was 52.5% pre-neutralized and the initiator charge was increased to 15.5%, on a total monomer weight basis. EXAMPLE 2 This example illustrates the efficacy of the copolymers of this invention in altering calcium crystal formation and inhibiting scale formation in water treatment systems. Calcium carbonate crystal formation was tested by treating aqueous solutions of calcium carbonate (160 ppm Ca ++) with a second batch of copolymer 1 of Table I ("copolymer 1A", Table II, was made using, as the initiator, 10.8% hydrogen peroxide, 4.5% sodium persulfate and 5.6% ferrous ion) and control polymers selected from commercial polyacrylate (molecular weight 2,600 - 3,000), acrylic acid/maleic acid 70/30 copolymer (molecular weight about 4,500), a 50/50 maleic/acrylic acid copolymer (Copolymer 3, Table I), a sulfonate styrene/maleic acid 3/1 molar ratio copolymer and a sulfonate styrene/maleic acid 1/1 mole ratio copolymer. For each polymer sample, 0.1% active polymer solution (3 mls; 30 ppm polymer) was added to a bottle containing 20 mls of 0.1 M Na 2 CO 3 and 30 mls of 0.1M NaHCO 3 . Control blank samples were also prepared. One set of samples was equilibrated to 50°C in a water batch and a matching set of samples was equilibrated to 70°C. Solutions of CaCl 2 were equilibrated to either 50 or 70°C and a 50 ml aliquot (160 ppm Ca++) of CaCl 2 was added to each test bottle and the control bottle at the corresponding temperature. The bottles were inverted three times and left in the 50 and 70°C water baths overnight. The calcium carbonate crystals that precipitated from these samples were examined by scanning electron microscopy. A scanning electron microscope (J.E.O.L. Model JXA - 6400, obtained from Japanese Electron Optical Ltd., Japan) set at 15 KV, with 2,000X magnification, was used to examine calcium carbonate crystals formed after treatment of calcium carbonate solutions with experimental and control polymer. Photomicrographs of various crystal formations were prepared. Photomicrographs of crystals treated with the Copolymer 1A, the polyacrylate, the 70/30 acrylate/maleate co-polymer, and the 50/50 maleic/acrylic copolymer (copolymer 3, Table I) are shown in Figures 1-4. As can be seen by comparing Figure 4 (copolymer 1A of this invention) with Figures 1, 2 and 3, treatment of hard water with the copolymers of this invention significantly distorted crystal morphology, creating crystals having narrow points of attachment to nucleation sites. The small attachment site of these crystals is believed to create mechanical instability and, thereby, inhibit scale formation. The control polymers did not modify crystal structure in this manner. The control crystals shown in Figures 1-3 exhibit traditional calcite and aragonite morphology with some distorted calcite and deformed aragonite morphology observed. Stressed Calcium Carbonate Test High electrolyte and high pH conditions were used to quantitatively test the copolymers of this invention for copolymer effects on calcium carbonate threshold inhibition, dispersancy and surface adhesion. A series of samples containing 160 ppm Ca++ ions were prepared at each of three pH levels (9.2, 10.7 and 12.3), blanks were prepared without added polymer and test samples were treated with 0.2% solutions of active, experimental and control polymers (1 ml of solution = 10 ppm polymer in sample). To prepare the samples at pH 9.2, 20 mls of 0.2M Na 2 CO 3 solution, 30 mls of a 0.2M NaHCO 3 solution and 50 mls of a 4.0M NaCl solution were added to a 250 ml erlenmeyer flask and, where indicated, polymer samples were added to the flasks. For pH 10.7 samples, 45 mls of 0.2M NaCO 3 solution and 5 mls of 0.2M NaHCO 3 solution were used. A 100 ml aliquot of a CaCl 2 2 H 2 O aqueous solution (0.588 g/L) containing 160 ppm Ca++ ions was added to each flask. Flasks were stoppered and placed into a 50°C water bath for 48 hours. For the 12.3 pH samples, 50 mls of the 0.2M NaCO 3 solution was added to the flasks, polymer solutions were added and then 50 ml of a 4.0M NaCl solution containing 0.5M NaOH was added simultaneously with 100 ml of the CaCl 2 solution. Sample preparation was otherwise identical to that of the pH 9.2 and 10.7 samples. Samples were divided into three parts to measure effects of polymer treatment on calcium carbonate solution properties: (1) filtrate aliquot, (2) membrane aliquot and (3) adherent aliquot. "Filtrate" aliquots contained calcium stabilized (inhibited) by the polymers. This is representative of the "threshold effect" of the polymer. "Membrane" aliquots contained solid material collected on a filter membrane. This is representative of "dispersing effect" of the polymers. "Adherent" aliquots contained material which had been attached to the surface of the sample flasks. This is representative of the effect of the polymer on the "adhesive" properties of formed crystals. To separate samples into these aliquots, the samples were removed from the water bath and allowed to equilibrate to room temperature. Holding the stopper securely, the sample flask was gently inverted three times and immediately filtered through 0.45 µm membrane filters to prepare the "filtrate" aliquot. A total of 1 ml of 12% KCl and 5 mls from the side arm flask were pipetted into a 100 ml volumetric flask. An aliquot of 50 mls of 14% nitric acid was added and the flask was filled to volume with deionized water. (5 mls of the filtrate is equal to 1:20 dilution). To prepare the "adherent" aliquot, 50 mls of 14% nitric acid was poured into the sample flask and swirled to dissolve crystals. The contents of the sample flask were transferred into a 100 ml volumetric flask and the sample flask was rinsed with 30 mls of deionized water while transferring into the volumetric flask. A total of 1 ml of 12% KCl was added to the volumetric and it was filled to the mark with deionized water. To prepare the "membrane" aliquot, the filtration apparatus was transferred to another side-arm flask and 50 mls of 14% nitric acid was slowly poured through the f ilter membrane, and the membrane was rinsed with 20 mls of deionized water. The contents of the filtration flask containing the "membrane" aliquot were transferred into a 100 ml volumetric flask. The flask was rinsed with about 20 mls of deionized water while transferring into the volumetric flask, 1 ml of 12% KCl was added to the volumetric and it was filled to volume with deionized water. Samples were analyzed by flame atomic absorption on a Perkin Elmer Model 5100 Atomic Absorption Spectrophometer. Results reported in Table II are calculated as the percent of calcium in the sample aliquots. Polymer Inhibition of Calcium Carbonate at High ph and High Electrolyte Percent Calcium Polymer (Mw) Polymer Treatment Level mg/1 Adherent % Membrane % Filtrate % Polyacrylate (2,500) 12 11.3 87.3 1.1 18 6.9 92.6 0.2 24 6.9 92.2 0.6 30 7.3 91.9 0.5 Acrylate/maleate 70/30 (4,500) 12 21.7 18.4 59.5 18 24.0 10.7 65.0 24 5.9 9.5 84.3 30 6.6 7.1 86.0 Sokalan® CP-5 (18,599) 12 34.6 29.1 36.2 18 34.7 9.1 56.1 24 20.0 17.7 62.2 30 14.1 17.8 68.0 Sokalan® CP-7 (13,966) 12 40.7 6.2 53.0 18 26.8 8.7 64.4 24 17.4 7.2 75.3 30 9.7 6.9 83.3 Sokalan® CP-45 (22.850) 12 41.6 13.9 44.4 18 32.8 16.7 50.4 24 37.1 8.8 54.1 30 22.9 8.8 68.2 Copolymer 1A (2,600) MA/AA/AM 50/45/5 12 20.9 4.6 74.3 18 10.6 3.4 85.7 24 1.2 4.2 94.4 30 1.3 4.8 93.8 Copolymer 3 (2,672) MA/AA 50/50 12 19.2 11.4 69.2 18 12.1 6.4 81.3 24 8.5 6.9 84.4 30 3.5 12.2 84.1 Copolymer 9 (2,639) MA/AA/VP 50/45/5 12 29.3 5.0 65.5 18 16.5 5.4 77.9 24 7.2 6.0 86.6 30 1.0 7.9 91.0 Copolymer 10 (2,990) MA/AA/DA 50/45/5 12 25.3 6.6 68.0 18 14.6 4.3 81.0 24 9.3 5.0 85.5 30 1.7 3.5 94.7 Copolymer 11 (2,698) MA/AA/HPA 50/45/5 12 19.9 4.8 75.2 18 10.0 3.0 86.8 24 1.2 3.6 95.0 30 0.9 4.5 94.3 The results show that the copolymers of this invention comprising a nonionic monomer (e.g., acrylamide) provided better threshold inhibition, dispersancy and scale inhibition than a 50/50 molar ratio maleate/acrylate copolymer of this invention and the commercial polymer controls. The 50/50 molar ratio maleate/acrylate copolymer of this invention having a molecular weight of 2,672 showed acceptable levels of calcium carbonate inhibition and was superior to the commercial controls (i.e., acrylate/maleate at a weight of 4,500, the Sokalan polymers and polyacrylate). Such enhanced performance was observed under high electrolyte and pH conditions. Thus, the copolymers of this invention provide superior efficacy in controlling scale formation in water systems.

Claims: A method for treating water to inhibit scale formation, comprising adding to the water an effective amount of maleate copolymer having a weight average molecular weight of 500 to 7,000 and a residual total monomer content of less than 1% based on a maleate copolymer dry weight basis, wherein said maleate copolymer comprises at least 50 mole percent of maleate monomer, 10 to 50 mole percent of acrylic or methacrylic acid, and 1 to 10 mole percent of at least one nonionic monomer selected from the group consisting of acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dipropylacrylamide, and vinyl acetate, whereby the proportions of constituents add up to 100 mole % wherein at least 70 mole percent of the carboxylic acid charge on the maleate monomer is neutralized prior to aqueous solution polymerization, and the maleate copolymer is prepared with a redox catalyst comprising at least one initiator and at least one metallic ion. The method according to Claim 1 wherein the maleate copolymer has an average molecular weight of 2,000 to 4,000. The method according to Claim 1 wherein the maleate copolymer is present in the water in an amount of from 1 to 100 ppm.

Inventors: AUSTIN ANNE-MARIE B, BELCHER JAMES H, STANDISH MICHAEL L, AUSTIN, ANNE-MARIE B., BELCHER, JAMES H., STANDISH, MICHAEL L.
Applicants: NAT STARCH AND CHEMICALINVEST, NATIONAL STARCH AND CHEMICALINVESTMENT HOLDING CORPORATION
Application year: 1994
Main classification: C02F5
IPC codes: D21H17, C11D3, D21H21, B01F17, C02F5, C09D17